1. FIELD OF THE INVENTION
The present invention relates to a device for the projection printing of the masks of a mask set onto a workpiece, particularly onto a semiconductor substrate for the manufacture of integrated circuits.
2. DESCRIPTION OF THE PRIOR ART
In the production of integrated circuits it is necessary to subsequently image a number of masks having different circuit patterns on exactly the same area of the substrate. After development the photosensitive layer being exposed on the substrate serves for covering the substrate at desired points during chemical and physical process cycless, e.g. etching- and diffusing processes, carried out between the concsecutive images. In the manufacture of integrated circuits high demands are made to accuracy. Tolerable deviations of the consecutive images of the circuit patterns are, for example, less than 1 .mu.m (0.039 mil.)
In order to obtain such accuracy the circuit patterns on the mask are usually imaged on the substrate by means of a projection lens, e.g. reduced by the factor 10. Before exposing a substrate or a portion of said substrate which has already been provided with circuit elements it is necessary to align alignment patterns of the mask with repetitive accuracy and relative to alignment targets marked on the substrate by aligning the masks, or, if required, the substrate, in order to obtain the desired covering of the successive circuit patterns.
In a first processing cycle of a prior art method alignment targets are produced on the virgin substrate for each chip by means of a mask and subsequent etching or by means of a laser, the masks required for the subsequent images of the individual circuit patterns being aligned to said alignment targets. In this step and repeat method each chip is exposed individually.
It is also known in the art to image the circuit patterns for a majority of or for all chips to be produced together on the substrate.
For the alignment process the respective alignment target portions on the substrate and the areas on the mask defining the alignment patterns are imaged into one another by means of the projection lens, the relative deviation being visually or metrologically determined. Positioning commands for the alignment mechanism, e.g. for the coordinate stage, are deduced from the deviation. In the aligned position the alignment targets and the alignment patterns are conjugated to one another.
It is a common problem to both methods that the alignment patterns of the mask are projected on the substrate during exposure and expose on the substrate the photosensitive layer in the area of the alignment targets. When the exposed or unexposed, photosensitive layer portions have been developed and removed, a portion on the substrate corresponding to the alignment pattern is uncovered and the alignment target usually becomes useless or is entirely etched away during a subsequent etching process. The target cannot be used for the alignment of the following mask.
When a negative, photosensitive layer is used on the substrate it has become known to prevent the optical printing of the alignment pattern by pre-exposing the area of the alignment target on the substrate.
When, however, a positive, photosensitive layer is used, the exposed layer is removed after processing, and the alignment pattern is destroyed during etching. When imaging a mask, a new alignment target for the next mask must be printed on the substrate. Thereby a disadvantageous addition of errors of alignment of the individual masks is caused.